first draft of audio fft spectrum visualizer (#5348)

examples/audio/resources/fft.glsl

@@ -0,0 +1,32 @@
+#version 330
+
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+out vec4 finalColor;
+
+uniform vec2 iResolution;
+uniform sampler2D iChannel0;
+
+const vec4  BLACK = vec4(0.0, 0.0, 0.0, 1.0);
+const vec4  WHITE = vec4(1.0, 1.0, 1.0, 1.0);
+const float FFT_ROW = 0.0;
+const float NUM_OF_BINS = 512.0;
+
+void main() {
+    vec2  fragCoord = fragTexCoord*iResolution;
+    float cell_width = iResolution.x/NUM_OF_BINS;
+    float bin_index = floor(fragCoord.x/cell_width);
+    float local_x = mod(fragCoord.x, cell_width);
+    float bar_width  = cell_width - 1.0;
+    vec4  color = BLACK;
+    if (local_x <= bar_width) {
+        float sample_x = (bin_index + 0.5)/NUM_OF_BINS;
+        vec2  sample_coord = vec2(sample_x, FFT_ROW);
+        float amplitude = texture(iChannel0, sample_coord).r; // only filled the red channel, all channels left open for alternative use
+        if (fragTexCoord.y < amplitude) {
+            color = WHITE;
+        }
+    }
+    finalColor = color;
+}